Three independent methods have been brought to bear on this
problem - the evolution of close galaxy pairs, the evolution
of morphologically-disturbed galaxies, and the evolution
of early-type galaxies as a proxy for plausible merger remnants.
All three methods suffer from important systematic and
interpretive difficulties. Close galaxy pairs, even supplemented
with spectroscopy to isolate galaxies within
500 km s-1
of each other, remain contaminated by galaxies in the same
local structures. Some measures of morphological
disturbance are susceptible to this source of error
to a lesser extent, and a clear consensus on the meaning
of the different automated and visual measures of morphological
disturbance is yet to emerge. Early-type galaxies will
be the result of only a subset of galaxy mergers and interactions,
and the role of disk re-accretion and fading in driving early-type
galaxy evolution is frustratingly unclear.

Yet, despite these difficulties some broad features are clear.
Mergers between ~ L* galaxies are almost
certainly more frequent
at higher redshift than at the present day, but this does not
imply by any means that galaxy mergers are unimportant at z ~ 1:
indeed, it is possible that an average ~ L*
galaxy undergoes roughly 1 merger between z = 1 and the present day
([Le Fèvre et
al. (2000)]).
This is supported
by the substantial build-up in stellar mass in red-sequence and early-type
galaxies since z ~ 1. Substantial uncertainties remain and
important questions, like the stellar mass dependence of the merger
rate, or the fraction of dissipationless vs. dissipational
mergers, are completely open. Nonetheless, these
first, encouraging steps imply that the massive galaxy population
is strongly affected by late galaxy
mergers, in excellent qualitative agreement with
our understanding of galaxy evolution in a
CDM Universe.